The invention relates to so-called scanning tips in probes for scanning a measured object as are in particular necessary for scanning force microscopes and other scanning microscopes, so-called scanning probe microscopes. In such microscopes, the achievable resolution is primarily dependent on the fineness of the tip, i.e. the curvature or radius of the outermost tip being as small as possible. In detail, the invention relates to a novel method for producing such scanning tips, probes having scanning tips produced according to said method, and, finally, the use of such scanning tips in a scanning probe microscope.
In scanning probe microscopy, a scanning tip commonly picks up the interaction of a probe and a measured object, i.e. a specimen, and leads the result to a suitable electronic. For scanning the specimen, essentially two methods are used. On the one hand, the so-called static mode in which the probe is mounted on an oscillating beam or cantilever and is in a fine contact with the specimen (actually almost without any contact), and on the other hand, the dynamic mode, in which the probe is mounted on a mechanical resonator and oscillates closely above the specimen surface. In the static mode, the excursions of the oscillating beam are evaluated, in the dynamic mode, however, the modifications in frequency or amplitude of the oscillation. The modifications in frequency or amplitude can be caused by an acoustic coupling of the probe and the specimen, the so-called acoustic near-field microscopy, or by atomic forces between the probe and the specimen, having a considerably higher resolution reaching molecular and atomic resolution, so-called force microscopy or atomic force microscopy, commonly abbreviated as AFM. In all of these microscopes, the resolution achieved is dependent to a large extend on the fineness of the scanning tip, i.e. on the radius of the outermost tip, opposite to the specimen, being as small as possible.
It is therefore obvious, that rather expensive processes for producing such scanning tips are being developed and great care is required in order to be able of producing the extremely small tip radius desired. Examples can be found in the publications of O. Wolter et al.: xe2x80x9cMicromachined Silicon Sensors for Scanning Force Microscopyxe2x80x9d, published in the Journal of Vacuum Science and Technology B, Vol. 9(2), (March/April 1991), pages 1353 ff. Frequently, the tips are integrated into other components of the microscope, in particularly embodied as parts of the probe, requiring rather complex production procedures.
A method for producing a so-called cantilever for an AFM, i.e. an oscillatory arm, is e.g. described in Muramatsu U.S. Pat. No. 5,877,412 having a single crystalline quartz tip integrated into its end. Here, the scanning tip is completely integrated into the cantilever, i.e. is a part of the cantilever. In the above-mentioned patent the complex production method of such a probe is described in detail.
In Karrai et al. U.S. Pat. No. 6,006,594, a probe for an AFM is shown with its scanning tip comprising a tungsten tip glued onto a cantilever. Here, the scanning tip is produced as a separate component and then connected with another component of the probe, namely the oscillating cantilever, whose frequency modification represents the measured variable.
In the IEEE Journal of Microelectromechanical Systems, Vol 8(1) 1999, pages 65-70 T. Akiyama et al. show a process in which silicon tips are transferred onto a cantilever by means of a batch process.
Finally, in Rev. Sci. Instr., Vol 70, 1999, pages 2398 ff, G. Genolet et al. describe an interesting production method for entire cantilevers made from photosensitive resist with, in a way, the scanning tip being integrated in a photosensitive resistxe2x80x94cantilever for scanning probe microscopes. The overall object of the solution shown aims at the production of economical cantilevers, several of which are arranged in a plate holder of sorts. In case of the xe2x80x9cactivexe2x80x9d cantilever becoming damaged it is mechanically broken off and removed. Then the xe2x80x9creservexe2x80x9d cantilever is drawn out of the plate holder into the scanning position and activated.
The processes described for producing scanning tips onto cantilevers or adhering thereto have in common that they are more or less complex, and that presently their tips can only be produced in a vacuum, in clean rooms, in molds, or under similarly limited circumstances. This causes the production to be tedious and expensive. Processes in which the tips are retrofitted onto a previously, in a way finished cantilever present a somewhat facilitated method, however, the processes known for retrofitting tips (in a parallel process) onto existing oscillating elements or cantilevers are characterized such that the tips must be erected perpendicularly to the level in which the oscillating elements are positioned.
As indicated above, the object of the present invention is to provide a method for producing scanning tips and similar devices which is easier, and thus more economical, than conventional production processes. The retrofitting of scanning tips onto existing structures shall be possible as well. Here, no compromises should be made in the quality of the scanning tips or their suitability for high resolution measuring. The particular advantage of the method is its possibility to laterally mount tips onto existing structures, i.e., on the same level as existing structures in a parallel process.
Generally speaking, the invention comprises in a photosensitive resist being used as the material for the scanning tip, more generally, a material able to be photo-structurable is used and a tip having a radius suitable for scanning probe microscopes is produced by exposing such material and subsequently developing it. In the following, such scanning tips are called xe2x80x9cplastic tipsxe2x80x9d.
In the above-mentioned process according to Genolet, a mechanical forming for structuring occurs perpendicular to the surface of the substrate, the process according to the invention uses a directed xe2x80x9cdiagonally angledxe2x80x9d photo exposure for the forming process and the tip structure, in particular, is provided laterally at the existing element, e.g., the cantilever. In particular, the latter results in an advantage not achievable with conventional methods, as described further below.
Furthermore, the invention comprises a process for producing such plastic tips directly onto quartz cantilevers in a parallel process, preferably using the so-called SU-8 photosensitive resist as the tip material, based on an epoxy resin and utilized primarily in microtechnology, here mostly as a corroding mask.
Additionally, the invention relates to the plastic tips produced and their use, in particular in scanning probe microscopy.
The unconventional approach of the invention consists in a different purpose and a novel result of an intermediate step and the material of the special production technology utilized therefor, here the semiconductor production. Although being already indicated in the publication of Genolet et al., it is not consequently followed through, however. Primarily, the possibility of producing (or repairing) in situ is not recognized at all by Genolet, since a completely oppositional approach was selected, namely the one of a shiftable reserve. xe2x80x9cIn situxe2x80x9d means here that the plastic tips can be formed onto an already existing (mass-produced) structure.
Thus, a particular advantage was not utilized at all by Genolet et al., namely that the processing of the photosensitive resist material can be performed in a normal environment, i.e. without a vacuum, highly clean room, or similar restrictions.
Another, rather general advantage is the enormously high technical standard of production processes presently achieved for microelectronics. This technology and the facilities required here are common and, thus can be reasonably purchased in high quality.
The invention can easily be adjusted to different conditions and can be modified in a simple manner. Therefore, it can be used for the production of scanning tips and for the mounting of additional tips onto a probe as well. On the other hand, the invention can also be used to produce or repair micromechanical sensors or probes that are used in other devices than scanning probe microscopes, e.g., in profile meters and measuring scanners. In the latter, an extremely small tip bending radius is not necessarily deciding, but frequently an optimally controlled geometry of the tip. This can easily be performed in the process according to the invention by embodying the mask accordingly, i.e. its shape and size, and, accordingly, by choosing the angle at which the exposure of the photo-structurable material occurs.
The invention is particularly suitable for the production of high-end cantilevers by retrofitting the tips onto comparatively inexpensive, mass-produced tuning fork quartzes.
Due to the possibility of a lateral mounting at the cantilever, the tip can additionally be already mounted when the cantilever, together with a multitude of identical ones, is still xe2x80x9cin situxe2x80x9d in the wafer, again allowing precise and inexpensive mass production.
Additional uses and embodiments result from the following description of the exemplary embodiment and from the claims.